Cellular telephone systems are becoming a significant part of the telecommunications network worldwide. These cellular telephone systems are typically implemented using a radio technology in which there are a plurality of base stations positioned throughout a geographic region. As a user of the network moves around within the geographic region, the mobile or portable telephone establishes communications channels with the base stations. The base stations are connected to the existing telephone network, and thus provide access by the mobile or portable telephone to a worldwide telecommunications system.
Radio terminals in cellular networks are known popularly as portable terminals or mobile terminals. The portable terminal typically refers to a handheld device that can be carried from place to place. A terminal which is mounted in an automobile or other vehicle is typically referred to as a mobile terminal. In the present application, the term "movable terminal" will be used to refer to both types, and generically to any movable radio terminal used in a network similar to that described and claimed herein.
Frequently it is desirable to determine the location of a movable radio terminal as it is moved around within a certain area. For example, it would be desirable to determine the location of the movable telephone in case of a medical or safety emergency so that help may be dispatched.
One prior art system for locating radio terminals within a cellular radio network is described in UK Patent Application No. GB 2 215 932 A, published Sep. 27, 1989, entitled RADIO COMMUNICATION SYSTEM, by McCaughan, et al. According to the MoCaughan, et al.. application, a designated base station selects and instructs other base stations independently to send acknowledgement request messages to the movable radio terminal. The base stations independently measure the roundtrip delay, including estimating the time it takes the radio terminal to receive and decode the request, and generate and send the acknowledgement. Since the time that it takes for the radio terminal to receive and decode the request, and generate and send the response is typically much longer than the roundtrip radio signal propagation times, and is variable, the method described in the McCaughan, application does not give very accurate location information Furthermore, the messages from the base stations arrive at uncoordinated times at the radio terminal, either overloading the radio terminal, or increasing the variability in response time to the various base stations. Also, this reliance on messages initiated from the base stations could interfere with other functions being carried out with the movable radio terminal. Furthermore, the location system described in this UK patent application increases the messaging load on the system, and could cause the radio system to reach a capacity overload point sooner.
Accordingly, it is desirable to provide a more accurate system for locating movable radio terminals which does not unreasonably burden the radio communication network.